Mobile Optimization is a critical because nowadays people serf more on mobile. That why mobile optimization become critical. Here is some link related to mobile optimization. 

Test teh mobile friendly: https://search.google.com/test/mobile-friendly

• https://www.livechatinc.com/community/t/fix-the-google-pagespeed-ensure-text-remains-visible-during-webfont-load/1753/2
• https://onilab.com/blog/magento-google-pagespeed-insights-optimization-guide/
• https://css-tricks.com/google-fonts-and-font-display/

Compacting JavaScript code can save many bytes of data and speed up downloading, parsing, and execution time.

• https://www.oreilly.com/library/view/high-performance-drupal/9781449358013/ch02.html

Make fewer HTTP requests

Reduce HTTP requests through combined files and CSS sprites.

• https://drupal.stackexchange.com/questions/153169/how-to-make-fewer-http-requests
•  

https://www.drupal.org/project/views_slideshow/issues/3023135

Insert an image normally & after that edit with source - then put the <a href="https://www.youwebsite.com"> as below and save. After that the image can clickable.

<p><a href="https://www.youwebsite.com"><img alt="" height="325" src="https://www.youwebsite.com/sites/default/files/styles/max_325x325/public/inline-images/imagename.jpg" width="325" /></a></p>

Point:-

Using a AdvAgg module - make a correct setting:-

Advance CSS/JS Aggregation > Modification > Optimized Javascript Ordering > Check the 'Move all browser conditional JavaScript to the bottom of the group'

Other reading:-

Remove dead rules from stylesheets and defer the loading of CSS not used for above-the-fold content to reduce unnecessary bytes consumed by network activity.